1. Field of the Invention
This invention relates to the field of acoustic devices and methods used for measuring the level of water in a ground water well and more specifically an acoustical device that relies upon an echoing sonic frequency sound wave to reliably determine the level of water in a ground water well that may contain obstructions.
2. Discussion of the Prior Art
The ability to measure the level of water in a ground water well is of significant importance to those individuals who rely upon such wells for their drinking water. The most common requirement is to know the volume of water in the well to determine the inventory of water available for use and to determine appropriate usage rates. Additionally, since ground water wells are sealed from surface water runoff and its contaminants, fluctuations in water levels in ground water wells may be indicative of leakage into the well and subsequent ground water contamination.
Echo systems for through-air measurement of the level of fluid in a container are well known. Typically, such a system includes a transducer that emits a burst of acoustic energy to locate the position of the surface of a fluid in a container such as a storage tank. The level of the fluid within the storage tank is determined based upon the amount of time required for the acoustic energy to travel from the transducer to the surface of the fluid and back again. It has been recognized that there are various practical problems associated with these systems. For example, once such problem is the generation of multiple return signals caused by reflections from the walls of the containers and possible internal structures such as pipes. Multiple return signals will serve to confound the return signal detector as to which return signal is representative of the true fluid level. One attempt to overcome the multiple return signal problem is described in U.S. Pat. No. 5,131,271 “Ultrasonic Level Detector” issued to Haynes and Margison on Jul. 21, 1992. This patent discloses a device having various improved signal processing features that ignores the spurious signals caused by echoes by only accepting an echo having a signal strength exceeding a magnitude exceeding a specified threshold.
The device taught by Haynes and Margison is a system that uses ultrasonic energy and therefore is suitable only for relatively short distances between the transducer and the level of the fluid. For distances involved in ground water wells, which may exceed several hundred meters, ultrasonic energy is not suitable because of the attenuation of the ultrasonic signal over distance. Ultrasonic energy is readily absorbed and reflected by obstructions in a ground water well such as roots, ropes, pipes and wires. Ultrasonic energy has the advantage over short distances of having a faster ramp-up rate resulting in a cleaner pulse of emitted energy. However, over a long distance, an ultrasonic pulse will be reflected by a number of surfaces. The resulting return echo ends up being a jumble of signals which are difficult to segregate between true and false level indications. As a result, ultrasonic energy systems such as that disclosed by the Haynes and Margison system rely upon a sophisticated software program that adds to the expense of the system and may place it out of the reach of individuals reliant upon ground water wells for their drinking water.
What is required is an acoustic measuring system that is simple and inexpensive to acquire and operate and overcomes the problems associated with spurious echoes in an environment where the attenuation characteristics of ultrasonic signals preclude their use.